On-line analysis for total moisture of a material is critical to enable a process technician and/or plant operator to optimize processes for a wide range of applications, such as dust suppression, process control, achieving product specifications and material handling.
Moisture measurement using microwave methods are based is upon a relatively high dielectric constant of water in comparison to a dielectric property of a material to be analysed. When a microwave signal passes through the material, some of the signal is absorbed such that the amplitude (ie. power level) of the microwave signal is less at a receiver than that transmitted. An amount of attenuation of this signal is related directly to the dielectric constant of the analysed material.
To illustrate this method, common sand (silicon dioxide) has a dielectric constant of 4.2 and water has a dielectric constant of 80.4. A variable amount of water within the sand results in a large variation in the combined dielectric constant of the sand and water, which is then measurable by detecting a change in the microwave signal.
In addition to monitoring attenuation, velocity of the microwave signal is also effected by the dielectric constant of the material to be analysed with and without water. Increases in the dielectric constant slows the velocity of the microwave signal as it passes through the analysed material. This slowing of microwave velocity is proportional to a phase shift in microwave signal. Accordingly, velocity may be determined by measuring phase shift of the microwave signal.
GB 2,122,741 describes an apparatus for monitoring crushed coal. The apparatus monitors ash content and moisture content of the coal by respectively transmitting and detecting X-ray and microwave radiation. The microwave radiation amplitude is chopped at a low frequency of about 1.0 KHz, which is suitable for analysing a crushed sample such as coal. However, this apparatus is not well suited for determining moisture content of is other types of samples by on-line sampling methods.
U.S. Pat. No. 4,788,853 describes a moisture meter that also uses microwave signals at discrete discontinuous frequencies. This patent states that the number of frequencies required to perform the invention is not critical as long as sufficient data is generated.
AU 61689/90 describes an apparatus for determining moisture content in a sample of varying thickness on a conveyor belt. The microwave signals are also transmitted at discrete discontinuous frequencies within a selected range.
Although the above described apparatus may be useful for determining moisture content in a sample, these apparatus are nevertheless still prone to substantial errors or inaccuracies due at least in part to variation in sample depth or configuration. Also, transferring microwave technology to an on-line situation such as a conveyor belt poses many challenges.